Method and apparatus for explosive autofrettage

ABSTRACT

A method and apparatus for producing residual compressive hoop stress at the inner bore of thick-walled tubes by the use of explosive charges positioned along the axis of said tubes. The explosive charge may be placed by itself along the axis of the thick-walled tube or may be enclosed in a second inner tube placed inside the thick-walled tube with an energy transmitting medium filling the space between the two tubes. The explosive charge is detonated producing pressure which plastically expands the thick-walled tube and causes residual compressive hoop stress to be developed at the inner bore upon dissipation of the pressure and elastic contraction of the thick-walled tube.

tates te [191 Ezra et a1.

METHOD AND APPARATUS FOR EXPLOSIVE AUTOFRETTAGE [73] Assignee: Denver Research Institute, Denver,

[22] Filed: June I, 1971 [21] Appl. No.: 148,487

[52] ILLS. C1. 72/56, 29/1.11, 29/421 E [51] Int. Cl B2ld 26/08 [58] Field of Search ..29/1.11,1.1, 421R, 29/421 B; 72/56, 367

[56] References Cited UNITED STATES PATENTS 3,235,955 2/1966 Kunsagi 29/421 E 3,156,973 11/1964 Lieberman 29/421 E 3,661,004 5/1972 Lee et a1. 29/421 E 1,578,751 3/1926 Methlin 29/1.11

1,602,282 10/1926 Methlin.... 29/1.11

3,172,199 3/1965 Schmidt 29/421 E 3,140,537 7/1964 Popofi' 29/421 E [451 Aug. 114, 1973 10/1964 Glyman et a1 29/421 E 4/1969 Berman etal. 29/421 OTHER PUBLICATIONS Primary Examiner-Richard J. I-lerbst Attorney-Anderson, Spangler & Wymore [57] ABSTRAUI A method and apparatus for producing residual compressive hoop stress at the inner bore of thick-walled tubes by the use of explosive charges positioned along the axis of said tubes. The explosive charge may be placed by itself along the axis of the thick-walled tube or may be enclosed in a second inner tube placed inside the thick-walled tube with an energy transmitting medium filling the space between the two tubes. The explosive charge is detonated producing pressure which plastically expands the thick-walled tube and causes residual compressive hoop stress to be developed at the inner bore upon dissipation of the pressure and elastic contraction of the thick-walled tube.

12 Claims, 6 Drawing Figures g 32 4e 2e 4e 30 2 a4 SIY//// /7a), *1 1 HHHMZW PAIENIE mm x 4mm SHEET 1 [1F 2 32 4a 2s 4s INVENTORS ARTHUR A EZRA WILLIAM G. HOWELL HERBERT S. GLICK MICHAEL A. KAPLAN ATTORNEYS ll METHOD AND APPARATUS FOR EXPLOSIVE AUTOFRETTAGE This invention is directed to a new process designed to produce large residual compressive hoop stress at the inner bore of thick-walled tubes. The existence of such compressive hoop stress retards the expansion of radial cracks in tubes subjected to high internal pressures and therefore lengthens the useful life of the tubes. One particular area of usefulness is in the manufacture of large ordnance gun barrels. Other areas of potential use are in the manufacture of high pressure piping and forging dies.

There are several methods existing in the prior art for producing a compressive hoop stress at the inner bore of a tube or cylinder. For example, a cylindrical ring can be placed over the outer surface of a thick-walled cylinder as a shrink fit thereby loading the cylinder with compressive hoop stress. This technique is generally limited with regard to the magnitude of the residual stress that can be created at the inner bore of the cylinder. Another method of inducing residual compressive hoop stress is the use of autofrettage. This process consists of raising the pressure in the tube to a value higher than that necessary to initiate plastic flow at the inner wall of the tube, thus inducing plastic flow in the tube wall. The plastic flow in the metal progresses outward from the bore through the wall as internal pressure increases. When intemal pressure is released, the wall is left with a residual stress distribution such that the bore has a compressive hoop stress. The tube is then said to be autofrettaged. The tensile hoop stress at the inner wall, produced by application of normal operating pressures, will then be greatly reduced, resulting in retardation of crack growth under variable pressures.

Conventional methods of autofrettage processes have been used extensively to lengthen the operational life of gun barrels. Generally, in the conventional hydraulic autofrettage process, the inner bore of a thickwalled cylinder is subjected to fluid pressures produced by hydraulic pumps which are sufficient to produce radial plastic flow throughout the cylinder wall. A die is usually employed at the outer surface of the tube to restrict the deformation produced by the plastic flow of the wall material. The internal fluid pressure is then released and the cylinder wall unloads elastically. Residual hoop stress is thus developed at the internal bore because of the difference between the plastic hydraulic stresses and the elastic unloading stresses. In ordinary practical operations the conventional hydraulic autofrettage method requires hydraulic equipment and pumps capable of producing fluid pressures well above 100,000 pounds per square inch and constraining dies of considerable size and weight. Equipment of this character is very expensive and the method requires substantial capital investment and operating expenditures.

In the conventional mechanical autofrettage process an oversized conical mandrel is forced through the bore of the cylinder by a mechanical device such as an hydraulic press. The residual stresses produced by this method are similar to those produced by hydraulic autofrettage. This method requires the use of a large and expensive hydraulic press and a mandril of a material with exceedingly high compressive strength.

It has now been found in accordance with the present invention that the autofrettage process can be accomplished by using explosives to produce the required pressures inside the thick-walled tube. One simple method for explosive autofrettage is to place a linear explosive charge centrally along the longitudinal axis of a thick-walled tube. The tube is then immersed in water and the charge detonated. While this configuration will produce some residual compressive hoop stress, the stress levels will generally be below those obtained by conventional autofrettage processes. Therefore, a more sophisticated explosive tube configuration has been developed which will produce residual compressive hoop stress comparable to that obtained by conventional autofrettage.

In the method and apparatus of the present invention, a rapid burning explosive charge is placed along the longitudinal axis of a ductile tube which in turn is placed along the longitudinal axis of the thick-walled tube which is to be autofrettaged. The region between the inner tube, referred to as the radial piston, and the thick-walled tube is filled with an energy transmitting medium such as water which is confined axially by plugs and a restraining fixture. The explosive charge is then detonated and the explosion gases operate to expand the radial piston thereby greatly increasing the pressure of the energy transfer medium confined between the radial piston and the inner bore of the thickwalled tube. Through the correct choice of the explosive charge and the respective dimensions of the radial piston and the thick-walled tube, complete plastic flow can be induced in the thick-walled tube and substantial residual compressive hoop stress will be obtained at the bore of the thick-walled tube upon dissipation of the explosively created pressure.

It is therefore the principal object of the present invention to provide a novel method and apparatus for producing large residual compressive hoop stress at the inner bore of thick-walled tubes.

A second objective is the provision of a method of autofrettage which does not require expensive capital investment in equipment such as hydraulic pumps, constraining dies, or hydraulic presses.

Another object of the herein disclosed invention is to provide an autofrettage method and apparatus which will reliably produce residual compressive hoop stress comparable to that produced by conventional autofrettage methods.

An additional object is to provide an explosive autofrettage method and apparatus which will operate to produce predictable residual compressive hoop stress in a variety of thick-walled tubes.

Further objects of the invention forming the subject matter hereof are to provide a method of autofrettage which is fast, reliable, easy to practice, relatively inexpensive as compared to conventional methods and sufficiently versatile to be used in connection with a variety of thick-walled tubes.

Other objects will be in part apparent and in part pointed out specifically hereinafter in connection with a description of the drawings that follows and in which:

FIG. 1 is a perspective view partially sectioned and broken away of a basic explosive autofrettage configuration showing the thick-walled tube and the explosive charge in place;

FIG. 2 is a transverse sectional view of the configuration shown in FIG. 1 taken along section line 2-2;

FIG. 3 is a diametrical section of an alternative type of autofrettage configuration;

FIG. 4 is a transverse sectional view of the configuration shown in FIG. 3 taken along section line 4-4;

FIG. 5 is a graph of a typical curve showing residual compressive hoop stress as a function of distance from the inside bore of a tube explosively autofrettaged using the configuration shown in FIGS. 1 and 2', and

FIG. 6 is a graph similar to FIG. 5 of a typical curve showing residual compressive hoop stress as a function of distance from the inside bore of a tube explosively autofrettaged using the configuration shown in FIGS. 3 and 4.

Referring now to the drawings and in particular to FIG. 1 in which reference numeral 10 denotes a thickwalled cylindrical tube or barrel such as would be used for heavy ordnance guns, for cylindrical forging dies, or for high pressure piping. As used in this application the term tube is to be given a broader than normal definition in that it shall be used to denote and describe any element having a hollow interior, the interior and exterior surfaces of which cooperate with one another to define a wall of the same or variable thickness either circumferentially or axially or both. While it is understood that the herein described and claimed process can be used to autofrettage any thick-walled tube as defined, the processes and apparatus will be discussed simply for ease of reference, particularly with respect to their use in treating gun barrels.

The basic configuration shown in FIGS. 1 and 2 is one of the simplest which can be used in an explosive autofrettage process. Here an explosive line charge is placed along the axis of the gun barrel and detonated. The detonation will, of course, produce expansion of the gun barrel and some residual compressive hoop stress at the inner bore of the gun barrel will result. The configuration of FIGS. 1 and 2 utilizes end caps 12 fitted over the ends of the gun barrel to hold and align a line charge along the axis of the barrel. In the particular configuration shown here, a wood dowel 14 is placed through center holes 16 which are positioned on the axis of the gun barrel and forms the structure to support the line charge. The explosive line charge is provided by wrapping the wood dowel 14 with explosive sheet material 18 such as Detasheet. A single strand of Primacord, or several strands together, can also be used as the explosive charge by suitably holding them in place along the longitudinal axis of the barrel. The end plugs 12 are also provided with vent holes 20 to allow trapped air to escape from the inside of the tube prior to detonation, when the configuration is immersed in water. The vent holes are not required when air is used as the surrounding medium. The Detasheet can then be detonated by the provision of a Detaplug 22 and ordinary blasting cap 24 on the end of the wood dowel. The explosive charge is then detonated in the air or preferably the whole configuration is submerged in water and detonated.

In the autofrettage of gun barrels, whether it be conventional or explosive, the purpose is to produce compressive hoop stress at the inner bore of the gun barrel. This stress results from subjecting the interior of the gun barrel to pressures which are sufficiently high to cause radial plastic flow throughout the gun barrel wall. The pressure inside the barrel is then released and the barrel wall unloads elastically. The residual compressive hoop stress is developed because of the difference between the stresses developed during plastic flow and the elastic unloading stresses. It is therefore the purpose of the line charge in the form of the Detasheet wrapped around the dowel to produce the required pressure to cause radial plastic flow in the wall of the barrel.

In applicants experiments ten inch long gun barrels having an inside diameter of approximately 1.1 inches and an outside diameter of approximately 2 inches made from annealed 4340 steel, heat treated to a 34 to 36 Rockwell C hardness, were successfully autofrettaged by the explosive process. In these experiments the explosive charge consisted of one to two wraps of Type C Detasheet on a 54, inch dowel. This configuration of Detasheet results in a charge weight varying between approximately I to 3 grams per inch of length. It was found that these charge weights respectively gave between 1 and 8 percent maximum bore expansion. Since it has been found that only about 0.5 to 2.5 percent maximum bore expansion is sufficient to produce the compressive hoop stress, a Detasheet charge weight of approximately I to 2 grams per inch of length is all that is required to successfully autofrettage gun barrels of this size and steel type.

The graph of FIG. 5 shows a typical curve where the residual hoop stress is plotted as a function of radial distance through the barrel wall from the inside bore. Here the negative values of hoop stress represent compressive stress and positive values represent tensile stress. This particular curve is representative of developed stress in one of the above described gun barrels using the explosive autofrettage configuration shown in FIGS. 1 and 2. The data for this curve was obtained through a stress determination process well known in the prior art as the Sachs boring-out method wherein strain gauges are mounted on the outside of the gun barrel and the bore of the barrel is successively enlarged through incremental machining operations. After each boring cut, the strain' gauge measurements are taken and used in conjunction with equations developed for the Sachs method to calculate the hoop stress at the new bore face.

As shown by FIG. 5, the compressive hoop stress at the inner bore of a typical test gun barrel autofrettaged by the configuration shown in FIGS. 1 and 2, was found to be approximately 31,000 pounds per square inch. While this value of compressive hoop stress is considerable, gr'eater values are obtainable and desirable in many applications. In accordance with the present invention, it has been found that the desired greater values of compressive hoop stress can be explosively obtained with the hereinafter described alternative apparatus and corresponding method.

To obtain a significantly high value of compressive hoop stress at the inner bore of the barrel, it was found that the pressure-time history which is produced by the explosive configuration is of critical importance. In producing the maximum residual compressive stress, the pressure must be of sufficient magnitude to cause plastic flow throughout the barrel wall just prior to elastic unloading. If the pressure is too low, and the tube wall remains entirely elastic, there will be no residual stress produced. Also, if the tube wall is only partially in plastic flow, the residual stress will be of intermediate magnitude. However, not all pressure-time histories which produce full plastic flow throughout the barrel wall will produce satisfactory values of residual compressive stress. For high values of stress, it is necessary that the bore pressures not produce additional plastic flow during the elastic unloading phase of the process. The range of pressure-time histories for which additional plastic flow does not occur depends on the particular autofrettage configuration. For the configuration shown in FIGS. 1 and 2, it is required that (1) the pressure at the inner bore of the barrel just prior to elastic unloading does not greatly exceed the pressure necessary to produce plastic flow throughout the barrel wall, and (2) the pressure at the barrel bore decays sufficiently slowly. High energy explosives currently available do not generally produce pressure-time histories which satisfy these requirements. As a result the tube suffers additional plastic flow. The greater the extent of the additional plastic flow the larger is the loss of residual compressive hoop stress at the bore.

The configuration shown in FIGS. 1 and 2 can be modified, however, to produce high values of residual stress. In this modified configuration, a slow-burning explosive is used. The barrel may be completely or partially filled with explosive, depending on the energy density of the explosive. Barrel end constraints are re quired to contain the gaseous products of the burning process.

It has been found that the range of pressure-time histories for which no additional plastic flow occurs is greatly expanded with the use of the explosive autofrettage configuration shown in FIGS. 3 and 4. This is primarily due to favorable phase relationships between the pressure-time history and the motion of the gun barrel produced by the geometry of the configuration. As a result, pressure-time histories which give near maximum values of residual compressive hoop stress are easily produced.

Reference numeral 26 indicates a gun barrel of the same approximate dimensions and steel types as described in conjunction with FIGS. I and 2. High strength steel cylindrical end caps 28 are mounted at each end of the gun barrel to serve as restraining members. These end caps are of a diameter larger than the gun barrel and have openings 30 drilled.therethrough outside the diameter of the gun barrel and parallel to the longitudinal barrel axis. Long threaded rods 32 passing through the openings 30 and parallel to the longitudinal axis of the gun barrel are inserted and rigidly fastened with threaded nuts 34 to clamp the end plates and the gun barrel into one unit.

Central bores 36 of approximately the same diameter as the inner bore of the gun barrel are provided in the inner faces of both end caps. The end caps are then positioned so that the bores 36 and the inside bore of the gun barrel are axially aligned. Tightly fitted into the central bores are cylindrical plugs 38 having axially projecting circumferential rims 40 which extend snugly into the bore of the gun barrel and further tend to hold the end caps and gun barrel in correct axial alignment. One of the end caps and corresponding plug have a central hole 42 drilled therethrough along the longitudinal axis of the gun barrel and of a diameter adapted to tightly hold a cylindrical blasting cap 44 described below.

A ductile tube 46, known as the radial piston, approximately the same length as the gun barrel and of a diameter sufficiently smaller than the bore of the barrel to form an annular cavity therebetween, is placed inside the barrel with its longitudinal axis coinciding-with that of the barrel. The radial piston also fits inside the circumferential rims 40 of plugs 38 so that the rims tightly seal the cavity 48 between the radial piston and the bore of the gun barrel. In the configuration used by applicants, the radial piston was constructed of 304 stainless steel of% inch outside diameter and 1/16 inch wall thickness. Thus, a cavity 48 of approximately 0.15 inch thickness is formed between the piston and gun barrel. In applicants experiments, it was found preferable to fill this cavity with a liquid to act as an energy transfer medium and cause even distribution of the pressure within the bore. Residual compressive hoop stresses of intermediate values can be obtained without tightly sealing the cavity 48 to the surrounding environment thereby allowing free flow of the energy transfer medium out of the cavity upon detonation of the charge. However, better results are obtained as the degree of confinement is increased.

The explosive charge for this configuration is contained within the radial piston 46. Any type of explosive charge may be used so long as it produces a close approximation to a line charge. One simple way would be to use a strand of Primacord strung along the coincident axes of the gun barrel and radial piston. A perhaps better and preferred configuration is that used by applicants wherein the radial piston is filled with a'low detonation velocity explosive powder. This powder configuration is the one shown in FIGS. 3 and 4 with reference numeral 50 indicating the powder within the radial piston. A powder satisfactory for this type and size of gun barrel was found to be SWP-2 manufactured by the Trojan Powder Company and having a detonation velocity of approximately 9,000 feet per second. A strand of Primacord centered within the radial piston along its longitudinal axis was used as the ignitor for the powder. In turn, an ordinary blasting cap 44 which fits fairly snugly within central hole 42 and connects to the Primacord ignitor 52 serves as a detonator. With the size of radial piston described above, the resulting main charge is approximately 35 grams of explosive powder. The use of this charge resulted in an average residual deformation of approximately 2 percent which indicates that sufficient pressure is produced to give satisfactory values of residual compressive hoop stress.

FIG. 6 is a typical curve of the residual stress as a function of distance from the inside bore of the barrel obtained with the configuration shown in FIGS. 3 and 4. This curve is developed by using the Sachs boringout method in exactly the same way as described in conjunction with FIG. 5. As indicated by FIG. 6, the ra' dial piston type of configuration shown in FIGS. 3 and 4 produced a residual compressive hoop stress of approximately 75,000 pounds per square inch. This value of compressive hoop stress obtained with the radial piston configuration is more than twice the value obtained without the radial piston as shown in FIG. 5.

The sequence of events occurring in the use of the radial piston configuration for explosive autofrettage is as follows. The blasting cap leads 45 are connected to a suitable electric power source causing detonation of the cap and ignition of the Primacord ignitor. The ignitor rapidly starts the detonation of the powder explosive along the axis of the radial piston so that the explosive pressures act in an essentially radial direction. The large internal pressure, developed after detonation of the explosive, forces the piston to accelerate radially.

.The outward motion of the piston is accompanied by expansion of the internal gases and therefore a decrease in internal pressure. At the same time, the compression of the water results in rapidly increasing cavity pressures. As this process continues, the net force on the piston becomes negative and the piston is slowed down and brought to rest. The combination of high water pressure and low gas pressure acting on the piston at this time produces large negative radial accelerations which drive the piston inward and cause the process to reverse itself. The pistons motion is therefore oscillatory. The pressure in the cavity, with proper design, will be sufficient to produce total plastic flow in the outer tube. Because of energy dissipated through plastic work and an increase in the bore of the outer tube, the magnitudes of both cavity pressure and piston velocity decay rapidly with time. The internal stresses developed in the tube oppose the water pressure and eventually decelerate the barrel wall to rest. As the deceleration continues, the barrel wall begins to move inward. Elastic unloading ensues and residual compressive hoop stress is developed.

It is the pressure-time history at the bore of the barrel which primarily determines the degree to which residual compressive hoop stress will be developed in the barrel wall. The pressure-time history depends on the amount and type of explosive, the dimensions, density and yield strength of the material of the radial piston, the thickness of the energy transfer medium between the radial piston and the gun barrel, and the dimensions, density and yield strength of the gun barrel. It has been found that the critical parameters can be satisfactorily sealed for different sizes and types of gun barrels.

By using the radial piston configuration for explosive autofrettage, satisfactorily high values of compressive hoop stress can be obtained at the inner bore without excessive residual deformation. In applicants experiments, without using any type of constraining dies, the residual deformation has been kept as low as approximately 0.5 percent while still producing satisfactorily high values of residual compressive stress. This 0.5 percent of residual deformation compares favorably with typical deformations for conventional autofrettage process. Thus, it has been determined that the use of constraining dies in conjunction with applicants explosive autofrettage process is unnecessary.

What is claimed is:

l. The explosive autofrettage apparatus for dynamically introducing residual hoop stresses into unconfined thick-walled hollow workpieces which comprises: a hollow envelope formed from a ductile material sized to fit within the hollow interior of the workpiece in spaced relation to its inside surfaces; an explosive charge confined within the ductile envelope; firing means associated with the explosive charge operative upon actuation to detonate same; and, an energy transfer medium filling the void between the envelope and workpiece, said medium cooperating with the gaseous products of combustion generated upon detonation of the explosive charge to rapidly establish a slowly decaying differential pressure oscillating across the interface defined by the ductile envelope therebetween ofa magnitude adapted to flex the latter and induce substantial transient plastic deformation in the wall of the workpiece without appreciable permanent distortion, and said elements also cooperating to reduce the amplitude of elastic oscillation of the workpiece so as to minimize reyielding.

2. The explosive autofrettage apparatus as disclosed in claim 1 in which: the ductile envelope is tubular;

and, in which the explosive charge is arranged coaxially therein, the gaseous products of combustion and transfer medium coacting to make a radially expandable and contractible piston of the tubular envelope.

3. The explosive autofrettage apparatus as disclosed in claim 1 in which: the magnitude of the explosive charge is limited to that which will produce not more than 2.5 percent permanent distortion in the workpiece.

4. The explosive autofrettage apparatus as disclosed in claim 1 in which: the ductile envelope is shaped to define a sealed container adapted to permanently separate the explosive charge and its gaseous products of combustion from the transfer medium.

5. The explosive autofrettage apparatus as disclosed in claim 1 in which: sealing means bridges the space between the ductile envelope and workpiece, said means being adapted to confine the energy transfer medium to the void therebetween.

6. The explosive autofrettage apparatus as disclosed in claim 1 in which: the energy transfer medium is water.

7. The explosive autofrettage apparatus as set forth in claim 1 in which: the explosive charge and ductile envelope are shaped and adapted to cooperate with one another and with the energy transfer medium so as to induce a substantially uniform deforming pressure in the workpiece with respect to both direction and magnitude.

8. The explosive autofrettage apparatus as set forth in claim 1 in which: the magnitude of the explosive charge is limited to that which will leave the workpiece with not to exceed 2.5 percent permanent distortion.

9. The explosive autofrettage apparatus as set forth in claim 8 in which: the magnitude of the explosive charge is sufficient to produce at least approximately 0.5 percent permanent distortion in the workpiece.

10. The method of dynamically autofrettaging an unconfined thick-wa'lled hollow workpiece without significantly altering its shape or size which comprises the steps of: introducing an explosive charge into the hollow interior of the workpiece of sufficient magnitude when detonated to induce substantial plastic flow in its walls but of insufiicient magnitude to materially distort same permanently; isolating said explosive charge from the workpiece by surrounding same with an energy transfer medium; and, interposing a yieldable membrane between the explosive charge and the pressure transfer medium adapted to separate one from the other while cooperating therewith to prolong the decay of the deforming pressure resulting from detonation of said explosive so as to minimize reyielding while tending to maximize the residual hoop stresses produceable in the workpiece by a charge of a given magnitude as it unloads elastically at the conclusion of the treatment.

11. The method of dynamically producing residual hoop stresses in a thick-walled workpiece in accordance with claim 10 which includes: limiting the magnitude of the explosive charge to that which will produce not more than approximately 2.5 percent permanent distortion in the workpiece.

12. The method of dynamically producing residual hoop stresses in a thick-walled workpiece in accordance with claim 11 which includes: using a sufficient explosive charge to induce not less than approximately 0.5 percent permanent distortion in the workpiece. 

1. The explosive autofrettage apparatus for dynamically introducing residual hoop stresses into unconfined thick-walled hollow workpieces which comprises: a hollow envelope formed from a ductile material sized to fit within the hollow interior of the workpiece in spaced relation to its inside surfaces; an explosive charge confined within the ductile envelope; firing means associated with the explosive charge operative upon actuation to detonate same; and, an energy transfer medium filling the void between the envelope and workpiece, said medium cooperating with the gaseous products of combustion generated upon detonation of the explosive charge to rapidly establish a slowly decaying differential pressure oscillating across the interface defined by the ductile envelope therebetween of a magnitude adapted to flex the latter and induce substantial transient plastic deformation in the wall of the workpiece without appreciable permanent distortion, and said elements also cooperating to reduce the amplitude of elastic oscillation of the workpiece so as to minimize reyielding.
 2. The explosive autofrettage apparatus as disclosed in claim 1 in which: the ductile envelope is tubular; and, in which the explosive charge is arranged coaxially therein, the gaseous products of combustion and transfer medium coacting to make a radially expandable and contractible piston of the tubular envelope.
 3. The explosive autofrettage apparatus as disclosed in claim 1 in which: the magnitude of the explosive charge is limited to that which will produce not more than 2.5 percent permanent distortion in the workpiece.
 4. The explosive autofrettage apparatus as disclosed in claim 1 in which: the ductile envelope is shaped to define a sealed container adapted to permanently separate the explosive charge and its gaseous products of combustion from the transfer medium.
 5. The explosive autofrettage apparatus as disclosed in claim 1 in which: sealing means bridges the space between the ductile envelope and workpiece, said means being adapted to confine the energy transfer medium to the void therebetween.
 6. The explosive autofrettage apparatus as disclosed in claim 1 in which: the energy transfer medium is water.
 7. The explosive autofrettage apparatus as set forth in claim 1 in which: the explosive charge and ductile envelope are shaped and adapted to cooperate with one another and with the energy transfer medium so as to induce a substantially uniform deforming pressure in the workpiece with respect to both direction and magnitude.
 8. The explosive autofrettage apparatus as set forth in claim 1 in which: the magnitude of the explosive charge iS limited to that which will leave the workpiece with not to exceed 2.5 percent permanent distortion.
 9. The explosive autofrettage apparatus as set forth in claim 8 in which: the magnitude of the explosive charge is sufficient to produce at least approximately 0.5 percent permanent distortion in the workpiece.
 10. The method of dynamically autofrettaging an unconfined thick-walled hollow workpiece without significantly altering its shape or size which comprises the steps of: introducing an explosive charge into the hollow interior of the workpiece of sufficient magnitude when detonated to induce substantial plastic flow in its walls but of insufficient magnitude to materially distort same permanently; isolating said explosive charge from the workpiece by surrounding same with an energy transfer medium; and, interposing a yieldable membrane between the explosive charge and the pressure transfer medium adapted to separate one from the other while cooperating therewith to prolong the decay of the deforming pressure resulting from detonation of said explosive so as to minimize reyielding while tending to maximize the residual hoop stresses produceable in the workpiece by a charge of a given magnitude as it unloads elastically at the conclusion of the treatment.
 11. The method of dynamically producing residual hoop stresses in a thick-walled workpiece in accordance with claim 10 which includes: limiting the magnitude of the explosive charge to that which will produce not more than approximately 2.5 percent permanent distortion in the workpiece.
 12. The method of dynamically producing residual hoop stresses in a thick-walled workpiece in accordance with claim 11 which includes: using a sufficient explosive charge to induce not less than approximately 0.5 percent permanent distortion in the workpiece. 